The primary winding of a transformer is generally disconnected from its power source prior to servicing. For safety's sake or for other reasons, it may be desirable or required that the secondary winding also be disconnected. A possible reason for this is the existence of a consumer power source connected to the transformer through the secondary leads. If power is supplied to the secondary winding from such a source, the power can be stepped up in the transformer and create a hazard for utility workers. In fact, utility workers are known to have been killed by coming into contact with such a stepped up voltage. Such sources may be unknown to utility workers and present a hidden danger. An example of such an unknown power source is backfeed from the generator of a recreational vehicle connected to the wiring of a house included in a transformer's service. There are other known possible sources.
In Australia, the State Electricity Commission of Victoria (SECV) is known to mount single-pole isolator switches underslung on a crossarm supporting a 230/400 volt three-phase, four-wire secondary conductors. This arrangement provides for the disconnection of both the primaries and secondaries for complete transformer isolation. The arrangement is unsatisfactory for North American utilities.
A multiple bushing connector apparatus for a padmount transformer is described in U.S. Pat. No. 4,787,855, issued Nov. 29, 1988. Use of the apparatus has the disadvantage of relying on releasable bushing connections between the transformer and external leads.